The conversion of synthesis gas to hydrocarbons is well known in the art, and the use of numerous catalytic processes have been suggested for optimal operation thereof. See for example Anderson R xe2x80x9cThe Fischer Tropsch synthesisxe2x80x9d, academic press, New York, 1984.
Many technologies exist to convert coal or other solid carbonaceous fuels to synthesis gas. The synthesis gas so produced consists predominantly of H2, CO, CO2 and CH4. The conversion of coal or other solid carbonaceous fuels to synthesis gas is also known as xe2x80x9cgasificationxe2x80x9d and typically is carried out in the presence of steam and oxygen in a gasifier such as a BGC-Lurgi slagging gasifier or a Texaco gasifier.
Synthesis gas so produced is reacted under Fischer-Tropsch conditions to produce gaseous and liquid hydrocarbons and oxygenates, containing amongst others, paraffins, olefins, alcohols and aromatics, with a variety of carbon chain length ranges and isomers which, in general, follow the well-known Anderson-Schultz-Flory distribution. For example, iron based catalysts are known for producing C2-C20 olefins, wherein the olefins comprise 60-70% by weight of the hydrocarbon products. Disclosures in the art, aimed at developing such catalyst systems and/or processes include, amongst others, the following patents: U.S. Pat. Nos. 4,604,375, 4,621,102, 4,618,597, 5,100,856, 5,118,715, 5,162,284, 5,185,378, GB 2 151 500A, and EP 0 446 035 A2.
The Fischer-Tropsch reaction is comprised of a series of polymerization reactions that can be represented by the generic chemical equations:
(2n+1)H2+nCOxe2x86x92CnH2n+2+nH2Oxe2x80x83xe2x80x83(paraffin production)
2nH2+nCOxe2x86x92CH3[(CH2)nxe2x88x923]CHxe2x95x90CH2+nH2Oxe2x80x83xe2x80x83(olefin production)
2nH2+nCOxe2x86x92CnH2n+2O+(nxe2x88x921)H2Oxe2x80x83xe2x80x83(alcohol production)
In addition, some catalysts that are active for the Fischer Tropsch reaction are also active for the water gas shift (WGS) reaction shown below. Fe based catalysts are notable in this regard.
CO+H2O⇄CO2+H2xe2x80x83xe2x80x83(water gas shift)
There are also a number of side reactions in which ketones, aldehydes, organic acids, branched hydrocarbons and cyclic compounds are formed. These compounds represent generally undesirable products, and efforts should be made to minimize production thereof A homologous series of predominantly straight chain hydrocarbons and alcohols with carbon number from 1 to greater than 100 may be formed, although substantial level of chain branching, particularly methyl branching is common for many catalyst systems.
Fe and Co based catalysts are of industrial interest. As indicated, iron based catalysts, as opposed to cobalt based catalysts are well known to be active for the water gas shift reaction (WGSR) and are thus able to convert synthesis gas with a low H2/CO ratio into liquid hydrocarbon products and CO2. This situation prevails when a hydrogen poor carbonaceous feedstock such as coal is gasified to synthesis gas.
It is an object of the invention to provide an improved process for producing longer chain alcohols and olefins from a raw carbonaceous feedstock which yields synthesis gas with a low H2/CO ratio, such as coal.
According to the invention there is provided a process for selectively producing linear alcohols, olefins and paraffins in a Fischer-Tropsch reactor, the process including:
producing synthesis gas containing hydrogen and carbon monoxide from a carbonaceous fuel;
modifying the ratio of hydrogen to carbon monoxide in the synthesis gas to a ratio that is at or above the overall hydrogen/carbon monoxide usage ratio in the reactor, but less than 2, typically between 1.2 and 2;
combining the modified synthesis gas with a recycled vapor product from the reactor to provide combined synthesis gas having a hydrogen/carbon monoxide ratio of greater than 2 and less than 3;
introducing the combined synthesis gas into the reactor;
reacting the hydrogen and carbon monoxide in the combined synthesis gas with an iron-based catalyst under Fischer-Tropsch conditions within the reactor; and
recovering linear alcohols, olefins and paraffins produced in the reactor.
The overall hydrogen/carbon monoxide usage ratio is a combination of the quantity of hydrogen consumed per mole of carbon monoxide in the Fischer-Tropsch reaction and the carbon monoxide converted to carbon dioxide and hydrogen by the water gas shift reaction.
The carbonaceous fuel is typically a hydrogen-poor carbonaceous fuel such as coal, petroleum coke or heavy residual oil.
The molar ratio of hydrogen to carbon monoxide in the synthesis gas may be modified by combining the synthesis gas with a second stream of gas containing hydrogen and carbon monoxide at a high molar ratio, typically of from 2 to 4.
The second stream of gas may be produced by:
a) passing synthesis gas through a water-gas shift reactor containing a catalyst that is active for a water-gas shift reaction but which is not active for hydrocarbon synthesis;
b) obtaining a paraffinic liquid hydrocarbon product from the reactor, passing the paraffinic liquid through a reforming reactor to convert the liquid to a reformed gas, and combining the reformed gas with the synthesis gas introduced in to the reactor;
c) utilizing a natural gas or refinery off gas stream or vapor stream from the Fischer-Tropsch reactor as feed to a steam reformer; or
d) a combination of any of the above methods a) to c).
A preferred iron-based catalyst for use in the above process is typically one in which the main iron phase is ferrihydrite and which includes Mn, Zn, Cu and K structural and chemical promoters.
For example, the iron based catalyst may, by mass, comprise:
35%-60% Fe, preferably 45%-60% Fe
0%-15% Mn, preferably 7%-15% Mn
3%-10% Zn, preferably 3%-7% Zn
0.5%-2% Cu, preferably 0.5%-1% Cu; and
0.5%-2% K2O, preferably 0.5%-1% K2O.
When bound with silica, the iron-based catalyst typically contains, by mass of the composition, 1%-30% silica.